Longitudinal data on four visits scheduled at 6 month intervals were available on a cohort of 1,827 homosexual men who were human immunodeficiency virus (HIV) seropositive at entry. To identify predictors of the rate of decline of CD4 T-lymphocytes, we used an autoregressive model that relates CD4 counts to predictor variables, while adjusting for previous CD4 counts. Significant predictors of steeper decline of CD4 counts were high CD8 count, low hemoglobin, low platelets, high serum IgA, high cytomegalovirus (CMV) antibody, and low HIV antibody. Using the fitted model, a subject with an initial deficit of 314 CD4 cells (median value of study sample) with respect to seronegative subjects and with average values in all other predictors is estimated to lose approximately 53 cells in a 6 month period (95% C.I. = 45-61 cells). Contrasting this estimate to the one obtained with similar methods in intravenous drug users, it is suggested that a faster rate of decline is present among i.v. drug users. This analysis provides evidence that several covariates in addition to previous number of CD4 counts have significant predictive power for estimating the decline in CD4 counts in HIV seropositive subjects.